INFLUENCE OF SOLVENT ON THE RATEOF CHLORINATION
March 5, 1959
must orient dipoles in its vicinity. There will be an orientation of solvent molecules in front of the moving ion and a relaxation to random orientation behind it. The steady state will correspond to a net excess of oriented dipoles behind the moving ion and a deficiency in front, due to the non-zero viscosity of the medium, and the ion therefore carries an electrostatic brake with itself in any polar solvent. This effect is a t least part of the reason why Aoq is not a true constant, as it would be if the Stokes model were valid. Finally, a suspected property of the picrate ion can be deduced from the conductance data. The ion is electrically asymmetrical; to first approximation, we may neglect (as a mutually cancelling vector sum) the dipole moments of the three nitrogroups. The net negative charge is, of course, distributed over the entire structure, but i t must be an unsymmetrical distribution, leaving a fairly strong dipole with the negative end a t the oxygen atom a t position 1. From the J values of systems 1-3, we
[CONTRIBUTION FROM
THE
1063
found d = 5.31. From the slope of Fig. 3, we obtain the smaller value of 3.18, if we assume that K A is given by equation 1. If, however, a dipole field is also involved, (1) should be replacedz3by In K A = a
+ ez/DkTa + pelDkTd2
(16)
where p is the dipole moment of the picrate ion and d is the distance parameter in the dipole term in the potential energy a t contact of anion and cation. The slope of the log K Avs. 1/D plot then is d log KA/dD-' = 0.4343(eZ/akT
+ pe/dakT)
(17)
Using the experimental value of 176 for the slope from Fig. 3 and it = 5.31, equation 17 gives 0.61 for the ratio p D / d 2 A (moment in debyes, distance in A.). This ratio appears to be quite reasonable; if the distance from the center of the cation to the center of the dipole in the picrate ion is set equal to 2 A., for example, we obtain a moment of 2.4 for the latter. (23) See discussion of sodium bromate, ref. 10.
ROME,ITALY
DEPARTMENT O F CHEMISTRY, UNIVERSITY
OF
CALIFORNIA, DAVIS]
The Influence of Solvent on the Rate of Aromatic Chlorination BY L. J. ANDREWS AND R. M. KEEFER RECEIVED JULY 31, 1958
I n ethylene dichloride the rate of toluene chlorination is extremely slow a t room temperature unless a polar catalyst is present. In the presence of added hydrogen chloride the reaction is first order with respect to each reactant and to the catalyst. The rate of simple bimolecular nuclear chlorination of toluene a t 25' varies with solvent in the order, ClCHzCHzCl.
